feat(radio): KCX mod support for TX16S MK3

[3djc]

This adds support for KCX mod for TX16S MK3

1. this brings an implementation for TX16S MK3: this uses AUX-USART3 connector on TX16S MK3

• a compile options is added: -DKCX_BTAUDIO=YES. Please note on TX16S MK3, this disables AUX2

The mod is a tad more complex than on TX15 as the audio connection points are on the non accessible side of the mainboard, so the mainboard needs to removed to solder audio wires

AUX-USART3	KCX Module
GND	GND (pin 4)
5V	+5V (pin 1)
RX	Link (pin 3)
TX	Connect (pin 11)
CS	Not connected

TODO: connection graphs

2. Usage

• the module start by default in pairing mode, but will connect automatically to an already paired headphone
• when BT headset is connected, the main speaker will automatically turn off
• volume commands are working like usual
• if you want to reset existing pairing, go to Radio Setup / General settings / Sound / push Pairing button toward bottom of screen